Audio frequency amplifier



May'19, 1959 R. E; WERNER 2,887,532

AUDIO FREQUENCY AMPLIFIER Filed Oct. 51, 1956 IIFVENTOR.

[Mam E Mme! ATZUBNEX' United States Patent OT AUDIO FREQUENCY AMPLIFIERRichard E. Werner, Medford Lakes, N.J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application October 31, 1956, SerialNo. 619,603

8 Claims. (Cl. 179-1) This invention relates to sound reproducingsystems,

and more particularly to audio frequency amplifiers havmg negativeoutput impedance characteristics for driving moving coil loudspeakers.

' Direct-radiator loudspeakers which are driven by an electrical signalapplied to a moving voice coil of the loudspeaker have an effectiveelectrical impedance that is determined by the electrical voice coilimpedance and the equivalent electrical impedance of the mechanicalimpedance of the loudspeaker. The electrical impedance of the voice coilis generally called the blocked voice coil impedance, since it ismeasured with the voice coil blocked or prevented from moving within themagnetic field of the loudspeaker. The mechanical impedance, which iseffectively in series with the blocked voice coil impedance, cannot beeasily treated unless it is converted to an equivalent electricalimpedance and when it is so converted it maybe called the reflectedmechanical ims pedance. In typical loudspeakers, the blocked voice coilimpedance is large compared to the reflected mechanical impedance.

speaker will result in a low resonant frequency, but will requireconsiderable electrical power to drive the system. If, however, theblocked voice coil impedance could be cancelled or eliminated, theamplifier output voltage would then be applied directly across thereflected mechanical impedance rather than through the blocked voicecoil impedance. The velocity of the voice coil driving the loudspeakerwould then be the exact replica of the signal voltage applied to theloudspeaker. The system would be substantially free of resonances, sincethe reflected mechanical impedance could be effectively damped, and thesystem would be substantially devoid of distortion caused by non-linearcompliance characterise tics of the loudspeaker. The cancellation of theblocked voice impedance may be effected by making the output impedanceof the amplifier driving the loudspeaker the negative of the blockedvoice coil impedance of the loudspeaker. i It is therefore an object ofthis invention to provide an improved audio frequency amplifier fordriving a mov ing coil loudspeaker system which substantially reducesthe distortion introduced by non-linearity characteristics of theloudspeaker. It is another object of the invention to provide an audiofrequency amplifier circuit for driving a loudspeaker system having anegative output impedance to cancel or reduce the effects of the blockedvoice-coil impedance of moving-coil loudspeakers.

It is another object of the present invention to provide an audio.frequency amplifier having a negative reactance output impedance.

The acoustic output of a direct-radiator moving voiceblocked voice coilimpedance is in series with the reflected mechanical impedanceintroduces several problems. The reflected mechanical impedance iseffectively a tuned circuit and thus has a resonant frequency, oftenwithin the audio frequency band. This resonance intro- In accordancewith, the invention, an audio frequency signal amplifier is made topossess an output impedance that is, in effect, a negative'resistauce,and a negative inductance in series to compensate for the positiveresistance and positive inductance of the blocked voice coil impedanceof a loudspeaker which is driven by the amplifier. Such output impedanceis provided by a negative impedance feedback circuit which includespositive current feedback and negative voltage feedback from the outputcircuit to the input circuit of the signal amplifier through abridgecircuit and a common feedback path. When the bridge circuit is nearbalance, there will be a net negative feedback at audio frequencies, anda stable negative output impedance for the amplifier, including negativeresistance and negative inductance, is provided. Positive feedback maybe applied over a portion of the amplifier duces non-linearities in theacoustic output signal of the loudspeaker since the reflected mechanicalimpedance generally cannot be efiectively damped because of the blockedvoice coil impedance in series with it. Even with the input terminals tothe voice 'coil shorted, the mechanical resonance is usually stilltroublesome.

Variation in the value of the reflected mechanical irnpedance withfrequency also introduces certain non-linearities. At frequenciesabovethe resonant frequency of the reflected mechanical impedance,.the valueof the reflected mechanical impedance is controlled largely by the massof the moving system, whereas at frequencies below the resonantfrequency it is controlled largely by the stiffness of the movingsystem. Since the stiffness is a non-linear function of the displacementof the cone of the loudspeaker, distortion will be introduced. Thenonlinearities and distortion caused by the resonances and non-linearoperation of the loudspeaker systemmay be reduced to some extent by theproper choice of cabinets or enclosures in which the loudspeaker ismounted. Also,

channel tov provide increased amplifier gain to more effectively cancelthe loudspeaker impedance and to reduce distortion in the amplifier byincreasing the net negative feedback of the negative impedance feedbackcircuit.

However, the invention will be further understood when the followingdescription is read in connection with the accompanying drawing, inwhich:

Figures 1 and 2 are schematic circuit diagrams of audio at amplifiercircuits constructed in accordance with the inan especially heavy coneand voice coil for the loudvention for driving moving-coil loudspeakers.

Referring now to the drawing and in particular to Fig;- ure 1, an audiofrequency amplifier includes, basically, a self-balancing phase inverterutilizing first and second triode amplifier tubes 10 and 12 to supplypush-pull signals to drive a pair of push-pull connected output tubes 14and '16, which, in turn, supply driving power to a loud speaker 86.Audio frequency signals to be amplified and reproduced are applied to apair of signal input terminals 18 and 20 and appear across a grid returnresistor 19. One terminal 20 is connected to ground or a point ofreferencepotential for the amplifier, and the other terminal 18 isconnected through a series input resistor 22 to the control grid 24 ofthe first amplifier tube 10. The anodes 26 and 28 of the first andsecond tubes 10 and 12 Patented May 19, 1959.,

assmsa are connected to a source'of operating potential, +13,

each through a separate load resistor 30 and 32, respectively, and theD.-C. circuit is completed by connecting the respective cathodes 34 and36 through individual cathode bias resistors 38 and 40 to ground for thesystem.

The anodes 26 and 28 of the phase invertertubes and 12 are connected tothe control grids 42 and 44 of the output tubes 14 and 16 throughcoupling capacitors 46 and 48. In order to supply an inversion signal tothe second tube 12, a pair of resistors 50 and 52 of properly relatedvalues, are connected in series between the control grids 42 and 44 ofthe output tubes 14 and 16, and the junction of the resistors isconnected to the control grid 54 of the second tube 12. The anodes 56and 58 of the output tubes 14 and 16 are connected to the centertappedprimary winding 60 of a push-pull output transformer 62, and operatingpotential is supplied to the output tubes 14 and 16 by connecting thescreen electrodes 64 and 66 thereof and the center tap 68 on the primary60 to the source of operating potential, +B. To complete the D.-C.circuit for the output tubes 14 and 16 the cathodes 70 and 72 areconnected through a common bias resistor 74 to ground for the system.The bias resistor 74 is by-passed by a capacitor 76 and the controlgrids 42 and 44 of the output tubes 14 and 16 are connected to groundthrough grid resistors 78 and 80, respectively.

Thus, audio frequency input signals applied to the input terminals 18and 20 are amplified and inverted in the phase inverter tubes 10 and 12and applied as push-pull signals to the control grids 42 and 44 of theoutput tubes 14 and 16. Output signals developed by the output tubes 14and 16 are applied through the transformer secondary 82 and aresistance-capacitance network, which will be more fully described andexplained hereinafter, to the moving voice coil 84 of the loudspeaker86. .The operation of such an audio frequency push-pull power amplifieras so far described is well known and further description of itsoperation and characteristics are not necessary other than to state thatby careful design the distortion characteristics of the amplifier may bereduced to a reasonably low value.

As previously noted, the equivalent impedance presented to an amplifierby a direct radiator moving-coil loudspeaker, such as the loudspeaker 86in Figure 1, is rather complicated and includes components of reflectedmechanical impedance and the blocked electrical impedance of the voicecoil. Even if the effective output impedance of the amplifier were zero,the reflected mechanical impedance of the loudspeaker, which is ineffect a rise to distortion and various undesired transient respouses.If the amplifier can be made to possess an output impedance that is thenegative of the blocked voice coil impedance, the voltage across thereflected mechanical impedance of the loudspeaker can be made to be theexact replica of the signal voltage applied to the amplifier.

In accordance with the invention, a feedback circuit to provide thenegative output impedance is connected between the secondary 82 of theoutput transformer 62 and the control grid 24 of the first amplifiertube 10. The negative impedance feedback circuit is connected to provideboth positive current feedback and negative voltage feedback over theamplifier. This is accomplished by connecting first and second bridgeresistors 90 and 88 in series across the secondary 82 and a third bridgeresistor the pair of bridge resistors 88 and 90 is connected to thecontrol grid 24 of the first tube 10 through a bass compensation networkcomprising a capacitor 94 in series with a resistor 96, the action ofwhich will be more fully explained hereinafter.

As can be seen from an inspection of the output circuit shown in Figure1, the resistance-capacitance network and the voice coil of theloudspeaker 86 form a bridge circuit, the first arm of which is thebridge resistor 88, the second arm is the bridge resistor 90 togetherwith the resistor 102 and the series capacitor 104 connectedthereacross, the third arm is a third bridge resistor 92, and the fourtharm is the voice coil 84 of the loudspeaker 86. The output signal of theamplifier is applied to one diagonal of the bridge circuit by connectingthe secondary winding 82 of the transformer 62 to two opposite cornersof the bridge circuit, that is, the junction of the bridge resistor 88and the voice coil 84 and the junction of the bridge resistors 90 and92. The negative impedance feedback circuit is connected across theother diagonal of the bridge by connecting the junction of the bridgeresistors 88 and 90 to the control grid 24 of the first tube 10, and byconnecting the junction of the third bridge resistor 92 and the voicecoil 84 to ground for the system.

The output impedance characteristic of the amplifier may be bestunderstood by the use of mathematical equations. Let Z equal theimpedance in the first bridge resistor 88; Z the impedance of the secondbridge resistor 90; Z the overall impedance of the third bridge resistor92 and the resistor 102 and series capacitor 104 connected thereacross;Z the blocked impedance of the voice coil 84 of the loudspeaker 86; Athe open circuit volt age gain of the basic amplifier; the minus signindicating a phase reversal, Z 'the' output impedance of the basicamplifier; A the open circuit voltage gain with the negative impedancefeedback circuit connected; and Z the output impedance with the negativeimpedance feedback circuit connected. If Z +Z Z and the resistance valueof the input resistor is sufificiently high to not effectively load thenegative impedance feedback circuit, which will normally be the case, itthen can be shown mathematically that 1 zl+zl The output impedance, thenwith the negative impedance feedback circuit connected can be shown tobe Z Z A 1+ 2 ZgA 1 1+ z If the bridge circuit then is balanced againstthe blocked voice coil impedance (Z and A is very high I 25-23 (a)Utilizing equations (2) -and (3), it can then be shown that 92 betweenthe secondary 82 and the voice coil 84 of the Thus, if the bridgecircuit is balanced against the voice coil impedance and the voltagegain of the basic amplifier is very high, the output impedance of theamplifier is the complement or the negative of the voice coil impedance.The feedback voltage at the junction of the bridge resistors 88 and 90is a summation of a negative feedback voltage developed from the voltageon the ungrounded side of the voice coil 84 and a positive currentcontrolled feedback voltage determined by the current flowing throughthe ,third bridge resistor 92. There will be no net feedback since thebridge circuit is balanced. However,

y it will be noted, that the reflected mechanical impedance of theloudspeaker was not considered in these equations. The reflectedmechanical impedance when the voice coil is allowed'to movewilfunbalanceithe bridge and-"provide a net negative feedback withoutaltering the output impedance of the amplifier. This. may be noted fromEquation 2. where Z' the output impedance of the amplifier with thenegative impedance feedback circuit connected, is seen to be in no waydependent upon theimpedance in the fourth arm of the bridge circuit inwhich the voice coil is connected.

The inductive component of the blocked voice coil impedance of theloudspeaker 86 is not an ideal inductance, and may be termed an'impureinductance in that it has resistive components caused by hysteresis andeddy-current, losses associated with the magnetic structured theloudspeaker 86; Accurate and complete cancellation of such an impureinductance is quite complicated and'generally not economically feasible,since the inductance Varies slightly. If a large percentage of theresistance of the'blocked voice coil impedance in cancelled, the Q ofthe inductance of the-blockedvoice coil impedance is raised, which mayresult in resonance problemswith the mass of the moving system in themiddle range of audio frequencies unless some reduction of the voicecoil inductance also is provided. Since it is not a simplematter toexactly cancel the blocked voice-coil inductance it 'is more practicalnot to attempt to cancel the blocked voice .coil impedance completely.Excellent results may be obtained if a substantial portion of theblocked voice coil impedance is cancelled, preferably on the order ofsixty to eighty percent, depending on the particular loudspeaker beingused.

The feedback circuit of Figure 1 includes a resistor 102 and capacitor104 connected in series across the second bridge resistor 90. Thecapacitor 164 provides that the feedback will make the amplifier outputimpedance appear to be a negative inductance, and the resistor 104 makesthe negative inductance appear impure in approximately the same ratiothat the inductance of the blocked voice coil impedance of theloudspeaker 86 is impure. Since the inductance of the blocked voice coilimpedance is not can celled 'to the same extent as its resistance, theblocked voice coil impedance will not be as effectively cancelled athigh audio frequencies as at loweraudio frequencies which willresult inreduced high frequency response. The capacitor 104 serves the additionalfunction ofincreasing the amplifier response at high audio frequencysignals to approximately compensate for this reduction in high frequencyresponse by decreasing the net negative feedback at these high frequencysignals.

At the low frequencies there is adecrease in the radiation efficiency ofthe loudspeaker, andthe input resistor 22 to the amplifier together withthe resistor 96 andthe capacitor 94 in the negative impedance feedbackcircuit provide a. bass boost circuit to compensate for thisdecrease inradiation efiiciency. The circuit has a time con.- stant to provide. afrequency varying loadingof the input circuit to the tube It) byincreasing the inputat lowfrequenci'es. as the'impedance of'thecapacitor 94 rises. The values are also chosen so that there isnegligible effect on the efiiciency of the feedbackcircuit providing thenegative output impedance for the lowest frequency of interest.

Belowithe resonant frequency of the loudspeakerthe reflected mechanicalimpedance is :.essentially controlled bygthestitfness of the movingsystem and if. the blocked voice coiLimpedance is not completelycancelled. the stiff.-

shown in Figure 1 is provided only through the feedback 6 circuitproviding the" negative output impedance. In order to 'moreeffectively-reduce the distortion in the amplifier proper, positivefeedback may be applied around the phase inverter through a capacitor 98and a resistor in series. This results in a high gain amplifier ofrelatively a few stages around which the feedback circuit providing thenegative output impedance is connected. As 'will be seen from theequations hereinbefore mentioned, the high gain of the amplifier resultsin more effective cancellation of the loudspeaker blocked voice coilimpedance, which gives rise to improved'frequency and transientresponse. The distortion of the amplifier itself is also reduced, sincethe gain of the amplifier is sufficiently high that the net negativefeedback of 'the negative impedance feedback circuit is increased to alarge value. The net negative feedback need not be sufficient to cancelthe positive feedback around the phase inverter stage since thedistortion of'the phase inverter stage can be made negligibly small bysuitable design.

The amount of positive feedback around the phase in verter stage will bedetermined principally by the values of the resistors 96 and 160 and thecapacitors 94 and in the negative impedance feedback circuit and thepositive feedback circuit, since the value of the input resistor 22 isverylarge compared to the reactance of the capacitor 94 and the resistor96 in the negative impedance feedbackcircuit. Since, the impedance ofthe bass boost circuit in the negative impedance feedback circuit,resistor 96 and capacitor 94, is a function of frequency, the value ofthe resistor 10% and the capacitor 98 in thepositive feedback circuitare made to have an impedance that is a similar function of frequency'sothat the positive feed-' back is essentially uniform at all frequencies.

Referring now'to Figure 2, an audio frequency amplifier embody-ing theinvention'includes many of the same components and configurations aspreviously described with respect to the circuit of Figure 1. However,instead of using a self-balancing phase inverter, as shown in Figure l,a voltage amplifier and phase splitter circuit are shown. The inputsignal applied to the terminal 18 is applied through the input resistor22 to the control grid 1% of a voltage amplifier tube 108. Operatingvoltage is supplied to the anode by connecting it through a loadresistor 112 to the source of positive operating voltage indicated at+B, and the cathode 114is connected to ground for the system and Bthrough a'cath ode resistor 116.

The amplified signal appearing at the anode of the voltage amplifier 108is'applied directly to the control grid 113 of a phase splitter tube129. In order to provide substantially balanced output signals fromthe'cathode 122 and the anode 124, an anode load or output resistor 126is connected between the anode 124 and the source of positive operatingpotential, +3, and a cathode load or output resistor 128 is connected inthecathode circuit. It will be noted that at the cathode load resistor128 is connected back to the cathode 114 of the voltage antplifiertubeltl and thence to ground through the-cathode resistor 116. Thisconnection supplies positive feedback from the phase splitter to thevoltage amplifieracrossthe cathode resistor-.116 to provide'an increasein overall gain for the amplifier for the reasons mentioned intheprevious descriptionof Figure 1.

The signals appearing at the anode andrcathode- 124 and 122 of the phasesplitter tube are applied through'the coupling capacitors 46 and 48 .tothe-control grids 42 and 44 of the. output tubes. The remainder of theamplifier circuit including the negative impedance feedback circuitconnections are similar to those previ* ously described with referenceto Figure 1.

Negative voltage feedbackand positive current feedback 'is suppliedthroughthe negative impedance feedback circuit in'the. same manneraspreviously described with reference to Figure 1. The negative outputimpedauce for the amplifier thus provided by the positive and negativefeedback appears inductive because of the first capacitor 104 connectedacross the first bridge resistor 90. A second capacitor 105 connectedacross the second bridge resistor 88 provides that the negativeinductance is impure in the same sense as the positive inductance of theblocked voice coil impedance of the loudspeaker 86 is impure. Thissecond capacitor 105 performs the same function as the resistor 102 inseries with the first capacitor 104 as shown in Figure 1. The resistorand capacitor 96 and 94 in the negative impedance feedback circuitprovides bass compensation in the same manner as hereinbefore described,and the positive feedback provides the desirable high gain for theamplifier.

In some instances it may be desirable or necessary to change thecharacter of the output impedance of the amplifier at extreme lowfrequency signals. For instance, if an inexpensive amplifier is used,low frequency transients may cause the amplifier to tend to drive theloudspeaker at a subsonic or a near subsonic signal and it is necessaryto reduce the power applied to the loudspeaker at these extreme lowfrequency signals because the available power from an inexpensiveamplifier is relatively low. This action may be accomplished byconnecting a third feedback circuit including a resistor 130 between theungrounded side of the voice coil 84 and the junction between the basscompensation resistor 96 and capacitor 94 in the negative impedancefeedback circuit. At frequencies of interest, that is, those desired tobe reproduced, the resistance of the resistor 130 is made sufiicientlyhigh as to have no effect on the balance of the bridge circuit for thenegative impedance feedback circuit. However, at the extreme lowfrequencies which may cause the conditions mentioned above, a highdegree of unbalance will result which produces relatively high negativefeedback. The large amount of negative feedback changes the outputimpedance of the amplifier in a positive direction at very lowfrequencies and also reduces the amplifier gain at these subsonic ornear subsonic signal frequencies. It should be noted that the use of theresistor 130 between the voice coil 84 and the input circuit may also beused, with exactly the same connections, in the amplifier circuit shownin Figure 1, with the same results.

An audio frequency amplifier constructed in accordance with the presentinvention to drive a direct-radiator, moving-coil loudspeaker ischaracterized by its virtually resonance and distortion freereproduction of audio frequency signals with extended low frequencyresponse. Thus, it is adapted for Wide application in sound reproducingfields requiring an accurate transducer of electrical signalsrepresenting sounds.

What is claimed is:

1. In a sound reproducing system for driving a direct radiatorloudspeaker having a moving voice coil, the combination of an audiofrequency signal amplifier having a signal input circuit, a signaloutput circuit for said amplifier including a transformer having asecondary winding, a first and a second resistor connected in seriesacross said secondary Winding, a third resistor, means connecting thevoice coil of the loudspeaker and said third resistor in series acrosssaid secondary winding, means connecting the junction of said voice coiland said third resistor to a point of reference potential for saidamplifier, circuit means connecting the junction of said first andsecond resistors to said input circuit for applying positive andnegative feedback over said amplifier and providing a negative outputimpedance therefor, and means including a fourth resistor and a firstfeedback control capacitor connected across said first resistor to makesaid negative output impedance substantially the complement of theelectrical impedance of said voice coil.

2. In a sound reproducing system, the combination in accordance withclaim 1, wherein positive feedback is applied over at least a portion ofsaid amplifier to increase the overall gain thereof to a relatively highvalue.

3. In a sound reproducing system for driving a direct radiatorloudspeaker having a moving voice coil, the combination of an audiofrequency signal amplifier having a signal input circuit, a signaloutput circuit for said amplifier including a transformer having asecondary winding, positive feedback means connected over at least aportion of said amplifier to increase the gain thereof to a relativelyhigh value, a first and a second resistor connected in series acrosssaid secondary winding, a third resistor, circuit means connecting thevoice coil of the loudspeaker and said third resistor in series acrosssaid secondary winding, said third resistor being connected between thevoice coil and the junction of the first resistor and secondary winding,circuit means connecting the junction of said voice coil and said thirdresistor to a point of reference potential for said amplifier, feedbackmeans connected between the junction of said first and second resistorsand the input circuit of said amplifier providing a negative outputimpedance for said amplifier, means including a fourth resistor and afirst capacitor in series across said first resistor to make saidnegative output impendance substantially the complement of theelectrical impedance of said voice coil, and means including a fifthresistor and a second capacitor serially connected in said feedbackmeans to reduce the loading of said signal input circuit at bassfrequency signals and provide bass frequency boost.

4. In a sound reproducing system for driving a direct radiatorloudspeaker having a moving voice coil, the combination of an audiofrequency signal amplifier having a signal input circuit, a signaloutput circuit for said amplifier including a transformer having asecondary winding, a first and a second feedback control resistorconnected in series across said secondary winding, a third feedbackcontrol resistor, circuit means connecting said voice coil and saidthird resistor in series across said secondary winding, said thirdresistor being connected between the voice coil and the junction of thefirst resistor and secondary winding, means connecting the junction ofsaid voice coil and said third resistor to a point of referencepotential for said amplifier, feedback circuit means connecting thejunction of said first and second resistors to said input circuit forapplying positive and negative feedback over said amplifier providing anegative output impedance therefor, and means including a high frequencyfeedback control capacitor connected across said first resistor to makesaid negative output impedance substantially the complement of theelectrical impedance of said voice coil.

5. In a sound reproducing system, the combination in accordance withclaim 4, wherein positive feedback is applied over at least a portion ofsaid amplifier to increase the overall gain thereof to a relatively highvalue.

6. In a sound reproducing system, the combination in accordance withclaim 4, wherein a fourth resistor and a third capacitor are connectedin series in said feedback circuit means to reduce the loading of saidsignal input circuit at low audio frequency signals and provide bassfrequency boost.

7. In a sound reproducing system for driving a direct radiatorloudspeaker having a moving voice coil, the combination of an audiofrequency signal amplifier having a signal input circuit, a signaloutput circuit for said amplifier including a transformer having asecondary winding, positive feedback means connected over at least aportion of said amplifier to increase the gain thereof to a relativelyhigh value, a first and a second resistor connected in series acrosssaid secondary winding, a third resistor, circuit means connecting thevoice coil of the loudspeaker and said third resistor in series acrosssaid secondary winding, said third resistor being connected between thevoice coil and the junction of the first resistor and secondary winding,circuit means connecting the junction of said voice coil and said thirdresistor to a point of reference potential for said amplifier, feedbackcircuit means connected between the junction of said first and secondresistors and the input circuit of said amplifier providing a negativeoutput impedance for said amplifier, means including a first capacitorconnected in circuit across said first resistor and a second capacitorconnected across said second resistor to control the frequency responseof the amplifier and make said negative output impedance substantiallythe complement of the electrical impedance of the voice coil, and meansincluding a fourth resistor and a third capacitor serially connected insaid feedback means to reduce the loading of said signal input circuitat bass frequency signals providing bass frequency boost.

References Cited in the file of this patent UNITED STATES PATENTS2,358,630 Fay Sept. 19, 1944

